1. Field of the Invention:
The present invention relates to a transmission ratio control apparatus of a toroidal type infinitely variable transmission, and in particular to such a control apparatus in which it is arranged to perform a accurate transmission control by making linear a relationship between the transmission ratio and a tilting angle of power rollers.
2. Prior Art:
A conventional toroidal type infinitely variable transmission is disclosed, for example, in U.S. Pat. No. 4,434,675.
Firstly, an arrangement of the toroidal type infinitely variable transmission exhibiting the principles thereof will be described with reference to FIG. 9. Reference numeral 1 designates a housing, and an input disk 2 and an output disk 3 are coaxially and rotatably mounted in the housing 1. The input disk 2 and output disk 3 are of an identical shape to each other and are disposed in line symmetry so that toroidal surfaces are formed having a circular cross section by opposing faces of the input and output disks 2, 3 cooperating with each other. And within a toroidal cavity formed by the toroidal surfaces of the input disk 2 and output disk 3, there are disposed with a pair of power rollers 4 and 5 which are respectively rotatably in engagement with both the disks 2 and 3.
These power rollers 4 and 5 are rotatably mounted on trunnions 6 and 7 respectively, and at the same time each of the power rollers 4 and 5 is supported pivotally with respect to a pivot axis O which is also the center of the toroidal surfaces formed by the input disk 2 and output disk 3.
And a viscous material having a large frictional resistance is coated on the contact surfaces between the input disk 2 and the power roller 4 and between the output disk 3 and the power roller 5, and rotatory power imparted to the input disk 2 is transmitted to the output disk 3 through the power rollers 4 and 5. The transmission ratio, that is, the speed changing ratio is changed by changing the tilting angle .theta. of each of the power rollers 4 and 5 by means of the trunnions 6 and 7. In other words, supposing that the power rollers 4 and 5 are in a horizontal condition with the tilting angle .theta.=0 as shown by the solid lines in FIG. 9, since a radius r of the input disk 2 at the contact position with the power rollers 4 is equal to that of the output disk 3 at the contact position with the power roller 4, and since these relationships are also true among the power roller 5, input disk 2 and output disk 3, the transmission ratio is 1:1. From this condition, when the power rollers 4 and are rotated by an (-.theta.) respectively clockwisely and counterclockwisely, the transmission ratio is increased with an increase in the tilting angle .theta.. In contrast, when the power rollers 4 and 5 are rotated by an angle (+.theta.) respectively counterclockwisely and clockwisely to positions indicated by chain lines in FIG. 9, the transmission ratio is decreased with an increase in the tilting angle .theta. in these angular directions. Accordingly, a desired transmission ratio can be obtained by controlling the tilting angle .theta. of each of the power rollers 4 and 5.
In this respect, in the conventional arrangement described in the foregoing, it is designed to move each of the trunnion in the pivot axis direction by operating hydraulic cylinders provided at opposite ends of each trunnion by a control valve thereby to tilt and pivot the power rollers. In this case the tilt and pivot movement of the power rollers is detected by a precess cam and the detected tilt and pivot movement is fed back to the control valve to control the transmission ratio to a desired value.
However, in the conventional toroidal type infinitely variable transmission described above, a relationship between the tilting angle .theta. of each of the power rollers 4 and 5 and the transmission ratio assumes, as shown in FIG. 10, a non-linear relationship of a hyperbolic characteristic. As a result, when it is applied, for example, to a transmission for transmitting the engine rotatory power of an automobile to the wheels, and when a servo control is performed by detecting the tilting angle .theta. of each of the power rollers 4 and 5, a gain of the transmission ratio becomes high in the speed increasing side of the transmission ratio as compared with the speed reducing side. Due to this fact, as shown in FIG. 11, the engine speed is forced to decrease undesirably, and thus a problem arises in which a comfortable ride is deteriorated.